The specular reflection properties of a tracheid cell of a wood object are well known. The primary response of light energy 20 striking a tracheid cell 22 on a surface 24 of a wood object 26 can be expressed by a simplified reflection 28 as shown in FIG. 1 The largest quantity of reflected energy is observed at a 45 degree angle with respect to the angle of incidence, and 90 degrees from the direction of the long axis of the tracheid cell 22 when there is no dive angle. A dive angle is present when the tracheid cells 22 align in a direction not parallel with the plane of the surface 24 of the wood object 26. A dive angle can be detected because the direction of the reflected energy varies with the angle of incidence. A. detailed discussion concerning the specular and diffuse reflective properties of a wood grain surface can be found in U.S. Pat. No. 4,606,645 of Matthews et al.
Conventional tracheid cell direction sensors 30a and 30b as shown respectively in FIGS. 2A and 2B employ various photosensor devices 32 that detect different reflections of light energy 20, from a light source 34 such as a laser, striking a wood surface 24 with a spot 18. An exemplary tracheid cell direction sensor is described in U.S. Pat. No. 5,252,836 of Matthews et al. Other methods include sweeping a light spot across the surface of a wood object.
One problem with conventional tracheid cell direction sensors is that they are not able to collect sufficient information across the surface 24 of a wood object 26 to satisfy some basic lumber grading requirements, nor are they fast enough at collecting data to perform applications that require real-time data acquisition and decision making. Another problem with conventional tracheid cell direction sensors is that the surface data they collect is not inherently or readily combinable with other location-sensitive data being taken across the face of a wood object 26 because conventional tracheid sensors collect data from a small surface coverage area limited to one spot per surface at any given time.
Another problem with conventional tracheid cell direction sensors is that the physical space required for the detection device is large, creating implementation difficulties in a production application where space is at a premium.